March 30, 2005: It's astounding how prophetic some science
fiction has been.

Back
in 1956, two years before NASA was even created, Hal Clement wrote
a short story called "Dust Rag" published in Astounding
Science Fiction, about two astronauts descending into a crater
on the Moon to investigate a mysterious haze dimming stars near the
lunar horizon. After discarding a wild guess that they were seeing
traces of a lunar atmosphere--"gases don't behave that way"--they
figured it had to be dust somehow suspended above the ground. In a
conversation remarkable for its scientific prescience, one of the
astronauts explains:

"âŚThe
[Moon's] surface material is one of the lousiest imaginable electrical
conductors, so the dust normally on the surface picks up and keeps
a charge. And what, dear student, happens to particles carrying
like electrical charges?"

"They are repelled from each other."

"Head of the class. And if a hundred-kilometer circle
with a rim a couple of [kilometers] high is charged all over, what
happens to the dust lying on it?"

The answer, given only by narrative description, is that electrostatic
charging caused the dust to levitate.

Well, guess what? Writer Clement was righter than he knew. It appears
lunar dust does levitate above the Moon's surface because of electrostatic
charging. And the first evidence came almost the way Clement had described.

In the early 1960s before Apollo 11, several early Surveyor spacecraft
that soft-landed on the Moon returned photographs showing an unmistakable
twilight glow low over the lunar horizon persisting after the sun
had set. Moreover, the distant horizon between land and sky did not
look razor-sharp, as would have been expected in a vacuum where there
was no atmospheric haze.

But most amazing of all, Apollo 17 astronauts orbiting the Moon in
1972 repeatedly saw and sketched what they variously called "bands,"
"streamers" or "twilight rays" for about 10 seconds
before lunar sunrise or lunar sunset. Such rays were also reported
by astronauts aboard Apollo 8, 10, and 15.

Above:
On the left are lunar "twilight rays" sketched by Apollo
17 astronauts; on the right are terrestrial crepuscular rays photographed
by author Trudy E. Bell. [More]

Here on Earth we see something similar: crepuscular
rays. These are shafts of light and shadow cast by mountain ridges
at sunrise or sunset. We see the shafts when they pass through dusty
air. Perhaps the Moon's "twilight rays" are caused, likewise,
by mountain shadows passing through levitating moondust. Many planetary
scientists in the 1970s thought so, and some of them wrote papers
to that effect (see the "more information" box at the end
of this story for references).

But without an atmosphere, how could dust hover far above the Moon's
surface? Even if temporarily kicked up by, say, a meteorite impact,
wouldn't dust particles rapidly settle back onto the ground?

Well, no--at least not according to the "dynamic fountain model"
for lunar dust recently proposed by Timothy J. Stubbs, Richard R.
Vondrak, and William M. Farrell of the Laboratory for Extraterrestrial
Physics at NASA's Goddard Space Flight Center.

"The Moon seems to have a tenuous atmosphere of moving dust
particles," Stubbs explains. "We use the word 'fountain'
to evoke the idea of a drinking fountain: the arc of water coming
out of the spout looks static, but we know the water molecules are
in motion." In the same way, individual bits of moondust are
constantly leaping up from and falling back to the Moon's surface,
giving rise to a "dust atmosphere" that looks static but
is composed of dust particles in constant motion.

You
can get some hands-on experience with the fountain model ... on top
of your head.

Rub an
inflated balloon on your hair, and then hold the balloon a few inches
away. Your hair will rise of its own accord to reach out toward the
balloon. Rubbing the balloon removes some of the electrons from your
hair, leaving your hair with a net positive charge. Your positively
charged hair is attracted to the negatively charged balloon.

Now watch what happens when you hold the balloon far away. This is
key: Your individual hairs spread out from one another and do not
immediately fall back to lie flat on your head. What's happened? When
the balloon was removed, each positively charged hair repels its positively
charged neighbor and some of your hair remains suspended--just like
dust on the Moon.

On the Moon, there is no rubbing. The dust is electrostatically charged
by the Sun in two different ways: by sunlight itself and by charged
particles flowing out from the Sun (the solar wind).

On the daylit side of the Moon, solar ultraviolet and X-ray radiation
is so energetic that it knocks electrons out of atoms and molecules
in the lunar soil. Positive charges build up until the tiniest particles
of lunar dust (measuring 1 micron and smaller) are repelled from the
surface and lofted anywhere from meters to kilometers high, with the
smallest particles reaching the highest altitudes, Stubbs explains.
Eventually they fall back toward the surface where the process is
repeated over and over again.

If that's what happens on the day side of the Moon, the natural question
then becomes, what happens on the night side? The dust there, Stubbs
believes, is negatively charged. This charge comes from electrons
in the solar wind, which flows around the Moon onto the night side.
Indeed, the fountain model suggests that the night side would charge
up to higher voltages than the day side, possibly launching dust particles
to higher velocities and altitudes.

Day side: positive. Night side: negative. What, then, happens at
the Moon's terminator--the moving line of sunrise or sunset between
day and night?

There could be "significant horizontal electric fields forming
between the day and night areas, so there might be horizontal dust transport,"
Stubbs speculates. "Dust would get sucked across the terminator
sideways." Because the biggest flows would involve microscopic
particles too small to see with the naked eye, an astronaut would not
notice dust speeding past. Still, if he or she were on the Moon's dark
side alert for lunar sunrise, the astronaut "might see a weird,
shifting glow extending along the horizon, almost like a dancing curtain
of light." Such a display might resemble pale auroras on Earth.

Stubbs and his colleagues are now hard at work on a host of fascinating
questions. For example, there are deep craters at the lunar poles
that never see sunlight. Would these craters have a strong surplus
of negative charge? Astronauts need to know, because in the years
ahead NASA plans to send people back to the Moon, and deep dark craters
are places where they might find pockets of frozen water--a crucial
resource for any colony. Will they also encounter swarms of electric
dust?